But in testimony before the Senate Committee on Veterans Affairs, Shinseki also defended the Department of Veterans Affairs' health care system as comparable to private medical providers.

Meanwhile, a top official from the VA's Office of Inspector General disclosed that his office is working closely with federal prosecutors as it examines allegations about manipulation of records at the Phoenix VA. He said the probe could result in criminal charges.

Also on hand to testify Thursday were the leaders of various veterans' organizations and advocacy groups. Most decried a perceived lack of integrity within the VA but also used the forum to call for more money and resources to solve root problems that contribute to VA health-care delays.

Several testified during the four-hour hearing that the overriding problem is patient access to physicians, and that veterans generally receive quality care once they get appointments.

Some committee members grilled Shinseki, the first to testify, and quoted VA documents showing a historic pattern of fraudulent record keeping. They also complained about a lack of transparency and accountability from his department.

Sen. Richard Burr, R-N.C., described a phone call that Robert Petzel, VA undersecretary for health, made to regional VA officials nationwide after the Phoenix VA director was placed on leave recently. Burr was told that Petzel told administrators that the removal was "political, and she's done nothing wrong."

"Why should this committee, or any veteran in America, believe that change is going to happen?" Burr asked.

In answer, Shinseki vowed to make changes in the VA if investigations substantiate allegations of flawed care and wrongdoing.

One of the veterans' advocacy group leaders said his organization had asked the Arizona Attorney General and the U.S. Attorney's Office to conduct criminal investigations of the Phoenix VA.

Sen. Richard Blumenthal, D-Conn., said he saw "ample evidence of criminal wrongdoing, including the destruction of records." He also suggested during the hearing that the FBI look into potential criminality within the VA.

Acting VA Inspector General Richard Griffin, whose office is investigating complaints about the VA, told senators that his team in Phoenix includes criminal investigators and is working with federal prosecutors from the U.S. Attorney's Office for Arizona and the Department of Justice's Office of Public Integrity.

The investigation is expected to be completed in August, he said.

Griffin was asked how many Phoenix patients on the so-called "secret list" of those who died awaiting care have been verified. Griffin said investigators have obtained "multiple lists (of deceased veterans), none of them identical."

The initial list his team received contained 17 names, not 40 as originally asserted, and so far investigators have not found that any of those deaths came from delays in care, he said.

Dr. John Daigh, assistant inspector general, added that the team has uncovered problems in the quality of care that may have resulted in patient harm. But he said attributing fatalities with those problems represents a "tenuous connection."

Griffin promised an exhaustive inquiry into record keeping, patient deaths and bonuses paid to VA administrators.

"I'm confident we have the resources and talent to complete a thorough review," he said.

Sen. Johnny Isakson, R-Ga., quoted from a previously undisclosed 2010 memo in which a VA administrator filled eight pages describing various methods that agency officials used to "game the system" by falsifying delays in medical care.

Petzel, who appeared before the senators with Shinseki, said he was aware of that memo and of ongoing problems involving some VA employees' attempts to manipulate data on patient access to enhance their job performances and bonuses.

"We have worked very hard, Senator Isakson, to root out these inappropriate scheduling practices, these abuses," Petzel said.

Shinseki and Petzel were asked repeatedly whether they had ever fired an employee for such fraudulent practices. Shinseki said 3,000 VA employees had been removed in 2013 - retired, transferred or terminated - for misconduct. He did not identify any fired for falsifying records on patient waits.

Sen. Patty Murray, D-Wash., was among those who upbraided Shinseki for failing to provide information that Congress requested and for failing to address "deep, system-wide problems" that have damaged the VA's credibility.

"The lack of transparency and the lack of accountability are inexcusable and cannot continue," she said.

Committee Chairman Bernie Sanders, I-Vt., anticipating the criticism, opened the meeting stressing that the VA has defenders who regard it as a model provider of health care and by noting that any major medical agency will have unhappy patients - especially one handling 85 million appointments annually.

Enrollment in the VA medical system has swollen because of the national recession, the increased number of military discharges as wars in Afghanistan and Iraq wind down, and more recently as the Affordable Care Act led some vets to enroll. In addition, those veterans being served are living longer and requiring more care as they age.

Sanders referred to a survey that showed roughly 95% of veterans are satisfied with their care and a magazine article that reported hospital errors as the third leading cause of death in the nation.

"There is no question in my mind that VA Health Care has problems, serious problems," Sanders said. "But it is not the case that the rest of health care in America is wonderful."

In written testimony, Shinseki emphasized his personal dedication and that of the department to serving veterans.

"VA provides safe, effective health care, equal to or exceeding the industry standard in many areas," his statement said. "We care deeply about every veteran. ... We can and we must do better. VA takes any allegations about patient care or employee misconduct very seriously."

Sen. John McCain, R-Ariz., who is not a member of the committee, also spoke angrily of the allegations in his home state and of frustrations veterans have expressed for years.

"Decent care for our veterans is the most solemn obligation our nation incurs," McCain said. "No one should be treated this way in a country as great as ours ... We should all be ashamed ... To date, the Obama administration has failed to respond in any effective manner."

The allegations of falsified waiting times and delays in care killing veterans not only prompted a VA Office of Inspector General investigation and but also spurred similar allegations from Department of Veterans Affairs employees in Colorado, Georgia, Illinois, New Mexico, Texas and Wyoming. Committee members said they now have similar information on VA facilities in at least 10 states.

In Phoenix, an interim director took over the VA Health Care System this week, replacing Director Sharon Helman, whom Shinseki placed on leave along with two other administrators.

The Phoenix VA Health Care System, which includes a hospital and at least a half dozen satellite clinics, serves about 80,000 veterans.

In the face of mounting criticism, Shinseki announced two weeks ago that every VA clinic in the country will undergo an audit of patient record-keeping procedures and practices.

But advocacy groups expressed concern Thursday that it might not be appropriate for the VA to conduct the audits and that an independent party should conduct them.

They also questioned how the audits could be accomplished within a few weeks, as apparently is planned.

The Obama administration has tapped Rob Nabors, White House deputy chief of staff, to help the VA with its internal audit. Shinseki said he welcomed Nabors' help in making sure veterans receive high-caliber health care in a timely fashion.

At one point during Thursday's hearing, Sen. Dean Heller, R-Nev., asked Shinseki directly: "Can you explain to me, knowing all this information, why you should not resign?"

Shinseki, a former Army general, responded that he was on a mission, already has made improvements to the VA, and "we're not done yet."

Daniel Dellinger, national commander of the American Legion, told senators he thought that Shinseki should resign, though the 2.5 million-member organization fully supports the VA.

Dellinger, in Phoenix two days ago for a nearly four-hour town hall that attracted more than 200 veterans and family members, said problems at the Phoenix VA were the "final straw" in a failure of leadership. He said new leadership was needed because of construction delays, cost overruns and lengthy patient waits at VA centers nationwide.

"When will things get better?" Dellinger asked.

VA officials in Arizona and the District of Columbia have denied intentionally falsifying patient wait-time information but have acknowledged confusion and discrepancies that Government Accounting Office investigations have uncovered in the past few years.

Burr listed numerous inspector general, GAO and other investigators' reports identifying long delays in medical care and inaccurate or bogus record-keeping as major VA issues.

"VA leadership should have been aware the system was facing a national scheduling crisis," Burr said. "Why were the national audits and statements of concern for the VA only made this month?"

Murray said she has pressed the department for years to eliminate the fraudulent reporting of patient access data. In 2012, she said an administrator told her the practice was so prevalent that each time a directive was put out to stop the practice, lower officials in the agency began tearing it apart to find loopholes.

"The standard practice of the VA seems to be to hide the truth in order to look good," Murray added. "That has got to change."

A groundswell of outrage, pain and frustration has emerged among veterans who have attended demonstrations and flooded members of Congress with complaints about flawed care, delayed appointments and other dysfunction in the VA health-care system.

After the inspector general completes its Arizona investigation, leaders of the House and Senate committees on veterans' affairs have vowed to conduct additional hearings. Yet Sanders decided in the face of mounting controversy to provide a forum Thursday for Shinseki and some organizations representing veterans organizations to speak about the issue.

Before the committee hearing, the Iraq and Afghanistan Veterans of America and Project on Government Oversight staged a joint press conference to announce a protection plan for VA whistle-blowers and to urge deeper investigations leading to solutions.

"Instead of leaders of the VA medical centers fixing problems with the system, they are choosing to fix the books," said Tom Tarantino, chief policy officer with the veterans groups.

During Thursday's Senate hearing, committee members cautioned against a rush to judgment.

Sanders stressed that all medical treatment networks have problems and urged the VA to be regarded in the context of 6.5 million patients - 236,000 seen daily - at 150 VA medical centers and more than 1,700 points of care employing more than 300,000 people.

Shinseki said the VA has enrolled 1 million additional veterans in the past few years and said he also is reviewing whether the department has adequate resources.